feat(mail-admin): read-only account view for mail.produktor.io (#74)

- admin/: Go stdlib-only HTTP viewer, lists accounts from
  config/postfix-accounts.cf with per-mailbox message counts (INBOX and
  total, same numbers doveadm reports) and storage; quota from
  dovecot-quotas.cf; Basic Auth from .env; offline tests vs fixtures
  (go test -race ./...)
- compose: mail-admin service, build admin/Dockerfile, publishes
  127.0.0.1:19945 / 172.17.0.1:19945; config and mail-data mounted :ro,
  no docker socket
- NPM proxy host 66: location /admin/ -> 172.17.0.1:19945
- README: account admin section

admin/testdata/accounts.cf

@@ -0,0 +1,7 @@
+# docker-mailserver postfix accounts (synthetic test data, NOT real hashes)
+info@produktor.io|{SHA512-CRYPT}$6$aaaaaaaa$fakehashinfo
+andriy.oblivantsev@produktor.io|{SHA512-CRYPT}$6$bbbbbbbb$fakehashandriy
+postmaster@produktor.io|{SHA512-CRYPT}$6$cccccccc$fakehashpostmaster
+postman@produktor.io|{SHA512-CRYPT}$6$dddddddd$fakehashpostman
+ano@produktor.io|{SHA512-CRYPT}$6$eeeeeeee$fakehashano
+malformed-line-without-pipe

admin/testdata/mail/produktor.io/ano/cur/1.fixtureano

@@ -0,0 +1,3 @@
+Subject: ano msg
+
+y

admin/testdata/mail/produktor.io/info/.Sent/cur/9.fixturesent

@@ -0,0 +1,3 @@
+Subject: sent item
+
+x

admin/testdata/mail/produktor.io/info/cur/1.fixture1

@@ -0,0 +1,4 @@
+Return-Path: <test@example.org>
+Subject: fixture one
+
+body

admin/testdata/mail/produktor.io/info/cur/2.fixture2

@@ -0,0 +1,4 @@
+Return-Path: <test@example.org>
+Subject: fixture two
+
+body body

admin/testdata/mail/produktor.io/info/new/3.fixture3

@@ -0,0 +1,4 @@
+Return-Path: <test@example.org>
+Subject: fixture three
+
+body

admin/testdata/mail/produktor.io/postmaster/new/1.fixturepm

@@ -0,0 +1,3 @@
+Subject: pm msg
+
+z

admin/testdata/quotas.cf

@@ -0,0 +1,3 @@
+# dovecot quotas (synthetic)
+info@produktor.io:quota=500M
+ano@produktor.io:quota=250M